Sampling tubes as described above are generally used for collecting substances contained in the air or a gas. In order to detect the amount of collected substances with a view to deriving therefrom the concentration of these substances in the air or the gas, the air or the gas is blown through the tube. The adsorption material contained in the sampling tube adsorbs a large number of substances from the air or the gas. In order to determine what substances were present in the air, the sampling tube is connected by one end, with interposition of a filtering device, to a source of inert carrier gas, such as for instance helium, and by the other end thereof to a gas chromatograph or another detection device, optionally with interposition of a cold trap. Then the filtered inert carrier gas coming from the source is pumped through the sampling tube to the detection device. When during that operation the sampling tube is heated to about 250.degree. C., the substances adsorbed in the adsorption material are liberated again. The detection device subsequently provides a quantitative indication of the substances contained in the carrier gas. The cold trap, if any is present, serves to temporarily freeze all substances released by the sampling tube. When the cold trap is switched off, all frozen substances are released in a short time and led to the detection device, which yields high and hence well detectable concentrations.
Such sampling tubes are used, for instance, to enable highly accurate determination of the air condition in submarines. This application is described in the article "Sampling of Submarine Atmospheres" by J. R. Wyatt, J. H. Callahan and T. J. Daley, in the SAE Technical paper Ser. No. 951656, pp. 1-6 (ISSN 0148-7191).
The sampling tubes can also be used in air pollution detector posts for measuring pollutions in the air in the street.
An important drawback of the known sampling tubes, also described already in the article mentioned, is formed by the pollution already present in the sampling tubes and which disturbs the measurements. In the above-mentioned article it is indicated that in the course of time unacceptable concentrations of benzene and toluene occur in the sampling tubes. In the case of detection involving an accuracy in the range of parts per trillion (ppt), the disturbance of the measurement caused by pollution in the sampling tubes is unacceptable. In the article mentioned, it is stated that in the future, experiments will be carried out with other adsorption materials in order to solve these problems. In the branch, apparently, the conviction is that the background disturbance of the measurements is caused by the adsorption material.
The insight underlying the present invention is that the pollution in the sampling tubes is not caused by the adsorption material itself but by the gas-permeable plugs between which the adsorption material is clamped. In the known sampling tubes, these gas-permeable plugs are made of glass wool. In order to avoid the glass wool being too brittle, it is produced while adding silicones. What is thus accomplished is that the glass wool, during the processing thereof, indeed remains glass wool and does not turn into powder. Upon heating of the sampling tubes, these silicones are liberated and disturb the measurement, and moreover the silicones adsorb substances that are liberated during the detection in an irregular manner, so that the gas chromatogram includes peaks that do not belong in it.